1. Field of the Invention
The present invention relates to amplifiers, in particular, relates to the linearization techniques of high frequency amplifiers.
2. Description of the Related Art
High frequency amplifiers are used in wireless communication systems, for example. High frequency amplifiers amplify high frequency signals, for example, modulation signals such as receiving signals, transmission signals, or the like.
A linear characteristic is one of the characteristics of a high frequency amplifier. As indices for indicating a linear characteristic of an amplifier configured to amplify modulation signals, a third order intermodulation distortion (IMD3) characteristic and a third order input intercept point (IIP3) are well-known, for example.
Japanese Unexamined Patent Application Publication No. 2006-180492 discloses an amplifier in which multiple gated transistors (MGTRs) are used. This amplifier is configured such that the characteristics of the main field effect transistors (FETs) differ from the characteristics of the auxiliary FETs in order to improve the linearity and reduce the third order intermodulation distortion thereof.
In order to configure the amplifier such that the characteristics of the main FETs differ from the characteristics of the auxiliary FETs, Japanese Unexamined Patent Application Publication No. 2006-180492 proposes to set the gate bias voltages supplied to the FETs (DC voltages applied to the gate ends) in the amplifier to different values, and to change the sizes of the respective FETs.
In the case where the gate bias voltages of the respective FETs are set to different values, DC cut capacitors are provided to the gate ends of the respective FETs so as to isolate the gate ends thereof from one another in a direct current (DC) way. However, if the respective FETs are equipped with the DC cut capacitors, the size of the amplifier (for example, the semiconductor chip size) increases, which makes it difficult to miniaturize the amplifier.
Meanwhile, in the case where the size of each of the FETs is varied, the unless power consumption of the FET is appropriately adjusted, the consumption current increases depending on the size of FET, which makes it difficult to lower the power consumption of the amplifier.